The general objective of these investigations is to study the physiologic maturation of the fetal lamb kidney and quantitate the contribution of the fetal kidney in compensating for various types of fetal distress. Specific investigations are designed to study: 1. The relative rates of maturation of renal blood flow, intracortical distribution of renal blood flow, and glomerular capillary flow utilizing radioactive microspheres. 2. The factors affecting the maturation of renal blood flow. 3. The influence of fetal hypoxemia on renal blood flow, gl merular capillary flow and glomerular filtration rate, and factors that may influence the response of the fetal kidney to hypoxemia. 4. The relation between the rate of filtraton in the cortical nephrons versus the juxtamedullary nephrons during gestation and immediately after birth utilizing a modification of the Hanssen Technique and C14 sodium ferrocyanide, and relate these findings to the distribution of glomerular capillary flow. These studies will provide valuable information leading to a better understanding of the fetus's response to intrauterine stress and the adjustment made during recovery. Furthermore, it will provide valuable physiological data which may stimulate new concepts regarding intrauterine therapy and management of the fetus during high risk pregnancy. Finally, these fetal studies will permit us to know where the developmental processs stands when a premature birth happens, and will help the survival of premature infants.